Optical fibers are used extensively in sensor arrays, in part because of the ease in forming the arrays. These fibers may have a core layer, e.g., core layer made of glass, an outer cladding layer, e.g., a layer made of glass over the core, and a protective buffer coating layer over the cladding. Unfortunately, these fibers and arrays have disadvantages. Often these sensors and arrays lack adequate protection when exposed to adverse environmental conditions.
Others have disclosed placing a protective coating or buffer layer on such fibers. See U.S. Pat. No. 6,563,996, disclosing optical fibers having a protective primary coating composition including a monomer with a pendant hydroxyl function group. Such protective layers do not necessarily protect sensor arrays including them, however. For example, optical fiber pressure sensor arrays that include such optical fibers on a wrapped mandrel may have entry and exit points exposed to potential damage. Such fibers and arrays need better protection.
These fiber arrays often include Fiber Bragg Gratings (FBG's) formed in the optical fibers through transparent buffer or coating layers, as by laser etching. U.S. Pat. No. 5,620,495 discloses writing FBG's in polymer-coated optical fibers without removing the polymer coating. Alternatively, FBG's may be written in the fibers by removing the buffer or coating layer over a short distance, e.g., a few millimeters, to form a gap, then forming Bragg gratings in the gap. The gap is then closed with a patch. See also U.S. Pat. No. 4,725,110 disclosing methods for forming FBG's within fiber optics.
Such methods may degrade the protective buffer of the fiber and may not provide adequate protection in the actual operating environment, either because the coating has been formulated for transparency, or has been stripped and recoated. This invention protects fiber sensor arrays with many FBG's in arrays such as those disclosed in U.S. Pat. No. 5,987,197 and up-coats each continuous fiber array length with a protective layer.